274 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 



may derive from the ancient salt deposits which have been impounded 

 from the sea is cyclic. 



The influence of wind-borne sodium has been fully discussed by 

 Sollas, Clarke, and Becker. There can be no doubt that it is rela- 

 tively unimportant. My own original correction was 10 per cent of 

 the river supply. Becker, by examining typical cross sections of the 

 isochlors, determined for the rainfall of western North America by 

 the United States Geological Survey, finds that an allowance of 6 

 per cent is sufficient. Sollas shows that these isochlors indicate that 

 but a small fraction of the sodium chloride of the American rivers 

 can be referred to this source. Clarke, by a somewhat different line 

 of attack, concludes that a correction of 7 per cent on the sodium 

 conveyed by the rivers of the United States is a maximum allowance. 

 Clarke further considers that a correction for sodium chloride carried 

 as dry dust is unnecessary. 



In a paper contributed by me to the Geological Magazine (May, 

 1900) I considered the possibility of oceanic sodium existing dissemi- 

 nated in the sedimentary rocks. Such sodium would be of course 

 cyclic. It was easy to show that, even on excessive estimates of the 

 occluded sodium chloride in such rocks, taken in conjunction with 

 their rate of removal by denudation, this source of supply to the rivers 

 is less than 1 per cent. Clarke reconsiders the question and finds 

 the allowance would not be more than 1 per cent. Three per cent is 

 regarded by Clarke as a maximum deduction for sodium artificially 

 supplied in modern times to the rivers. 



Oceanic salt deposits are not very abundant over the surface of 

 the earth, being generally confined to particular formations. That 

 they seriously affect the river analyses of all the great rivers of the 

 world is in the highest degree improbable. In any case if we deduct 

 all the chlorinated sodium from the river supply we must include 

 also all sea-derived sodium. If we effect this calculation, we obtain 

 an age of about 150 million years. I do not think it will be disputed 

 that this figure is in its nature excessive. 



There remains the possibility (d) that the assumed uniformity of 

 past and present conditions is illusory; in other words, that special 

 conditions now exist tending to bring about an abnormally great 

 river supply of sodium. 



The present is admittedly a period of large land exposure. This, 

 however, involves a fact which must be held in mind. At the pres- 

 ent time the land area actually draining into the ocean is about 39.7 

 millions of square miles. The total land area is, however, rather 

 over 55 millions of square miles. It follows that about 30 per cent 

 of the land area contributes nothing to the ocean. Or, again, the 

 areas which are classed as "rainless" — that is, which have less than 

 an annual rainfaill of 10 iircfees asnd have no nm~off— - are estimated as 



